236 GEOLOGY OF EASTERN WISCONSIN. 



burns red, and a specimen of clay and of pressed brick from Milwau- 

 kee, with the following result: 



Milwaukee Madison Milwaukee 



Clay. Clay. Brick. 



Silica 38.22 75.80 53.78 



Alumina 9.75 11.07 13.21 



Peroxide of iron 2.84 3.53 4.92 



Protoxide of iron 1.16 .31 .26 



Carbonate of lime 23.20 2.45 



Carbonate of magnesia 15.83 .17 7. 41' 



Lime(CaO) 3.24 .39 17.71 



Potash 2.16 1.74 1.54 



Soda 65 1.40 .92 



Water in composition 1.85 2.16 



Moisture.. .95 1.54 .19 2 



99.85 99.56 99.94 



1 Magnesia. * Loss on ignition. 



From these analyses it appears that there is even a larger quantity 

 of iron oxides in the Milwaukee clay, that burns white, than in the 

 Madison clay, that burns red, and that in the white brick there are 

 over five per cent, of iron oxides. It is also to be observed that the 

 Milwaukee clay contains a very large ingredient of lime and magne- 

 sia, of which the Madison clay contains but little. In view of these 

 facts Mr. Sweet suggests that the ingredients of the clay enter into a 

 combination somewhat analogous to some members of the amphibole 

 group in which the iron does not manifest itself as a coloring material. 



This view is confirmed by the hard and often semi-vitreous charac- 

 ter of the brick, and by the fact that the light color is developed at 

 the point of incipient vitrification. It is at least certain that the 

 light color is not due to the absence of iron, but to the manner of its 

 combination. 



The red clays, as has been already repeatedly remarked, are highly 

 calcareous, and from the close similarity of the brick formed from 

 them to the Milwaukee product it is evident that both undergo essen- 

 tially the same reactions. 



It is thought to be entirely safe to say that in quantity, quality, 

 convenience of situation and facilities for shipment, these clays are 

 unsurpassed on this continent. The superiority of the brick is uni- 

 versally acknowledged, and their beauty is a matter of general com- 

 mendation. The entire number annually manufactured is estimated 

 at 50,000,000, and the value of the product approaches half a million 

 dollars. The test and verdict of the recent great fires has been highly 

 favorable to brick as a building material, and there can be no doubt 



